This invention is drawn to a method and apparatus for the simultaneous supercritical fluid extraction (SFE) of analytes from a solid matrix and separation and analysis of such analytes by gas chromatography (GC). More particularly, this invention is drawn to a system for the simultaneous supercritical fluid extraction of samples from a solid matrix, sequentially absorbing such samples into a stationary phase in a thermal desorption modulator (TDM), and then releasing such samples at timed intervals into a gas chromatography column as a sharp concentration pulse by thermal desorption thereby allowing a set of high speed chromatograms, sampled from the extraction stream, to be generated while the SFE is proceeding.
Supercritical fluid extraction (SFE) is a method commonly used for sample preparation before analysis The fundamental mechanism of isolating chemical substances of interest from a matrix is based on the solubility of the substance in the extracting fluid. Since viscosities and solute diffusivities of supercritical fluids are similar to those of gases, and their solvating properties approach those of liquids, supercritical fluid extraction can be more efficient and faster than conventional liquid extraction because of more rapid mass transfer and better penetration into the sample matrix. The ease of solvent removal and low extraction temperature are important advantages of SFE when using fluids such as supercritical CO.sub.2.
Since analytically useful supercritical fluids usually have low boiling points and sample concentration can be easily achieved by reducing the pressure, on-line coupling of SFE with chromatographic methods should provide easy and reliable sample preparation and analysis Although many attempts have been made to couple supercritical fluid extraction to various chromatographic techniques, none of the methods reported have been successful in carrying out supercritical fluid extraction and chromatographic analysis simultaneously.
Stahl and co-workers coupled supercritical fluid extraction to thin-layer chromatography (TLC) and studied the solubility behavior of various compounds in supercritical carbon dioxide (Stahl et al., Fresenius Z. Anal. Chem. 1976, 280, 99-104). With this apparatus, the extracted substances were deposited on the TLC plate during extraction. However, chromatographic analysis was not started until the extraction was completed.
Coupling of supercritical fluid extraction to high performance liquid chromatography (HPLC) was first reported by Unger et al. (Unger et al., J. Chromatogram 1983, 282. 519-526), and later by others. In general, the system consisted of a constant-pressure pump to transfer the CO.sub.2 to a heated vial containing the sample, an injection valve and an analytical column. The injection loop was first loaded with the extracted substances. The chromatographic analysis then started after injection of the contents of the loop into the front of the analytical column.
Coupling of SFE to capillary supercritical fluid chromatography (SFC) was initially reported by Gmur et al. (Gmur et aI., J. Chromatogr 1987, 388, 143-150), and later by others (Levy et aI., J. Chromatogr. Sci. 1989, 27, 341-346). Interfacing between SFE and SFC was generally achieved by using either a sample injection valve or by cryogenically trapping the extracted sample onto the column head. The extraction and chromatographic analysis were carried out sequentially
Hawthorne et al. were the first and the leading investigators in coupling supercritical fluid extraction to gas chromatography (GC) (Hawthorne et al., J. Chromatogr. Sci., 1986, 24, 258-264). This technique was also demonstrated by many other investigators such as Levy et al. and Wright et al. (Levy et al., J. High Result. Chromatogr., 1990, 13, 418-421; et al., Anal. Chem., 1987, 59, 640-644). Since supercritical fluids decompress into gases under typical GC conditions, several interfaces based on standard GC injectors such as split/splitless injectors, programmed temperature vaporizer injectors, and cold trap injection systems have been described in the literature. As an alternative method, the extractant can be directly transferred onto the column through an on-column injection port. These methods are time consuming because the extraction and chromatography are carried out stepwise. Quantitative analysis depends on both completeness of the extraction and quantitative sample transfer. Furthermore, losing of lower molecular weight components during extraction, and peak shape deterioration for the components are problems commonly encountered in the coupled SFE/GC systems referenced in the prior art.
Using an on-column thermal desorption modulator for sample injection in GC has been reported by Phillips et al. (Phillips et al., J. Chromatogr. Sci., 1986, 24, 396-399). The device was a short section of the analytical column, configured such that the section temperature could be independently controlled apart from the rest of the column. A concentration pulse served as an injection and was generated by a thermal pulse which released retained substances. Thermal modulation, as an interface between SFE and SFC, was demonstrated by Mitra (Mitra et al., J. Chromatogr. Sci., 1990, 28, 182-185). Using an on-column thermal modulator for sample transfer between gas chromatographic columns was demonstrated by Liu et al. (Liu et al., J. Chromatogr. Sci., 1991, 29, 227-231).
While certain components utilized in the present invention are to be found in the prior art as referenced above, no application was found for the use of a thermal modulator as an interface between supercritical fluid extraction and gas chromatography to allow integration of these systems and achieve the simultaneous performance of the two in the extraction and analysis of samples.